1. Field of the Invention
This invention relates to acoustic imaging, and more particularly, to an apparatus and method for processing passive acoustic signals received on a horizontal line array using a reduced beam space processor.
2. Description of Related Art
Signal processing in underwater acoustics has been centered around the problem of detection and localization of a target or signal in an ocean waveguide. Detection and localization of a quiet target requires the use of an array of hydrophones as the array processing gain will enhance the signal-to-noise (S/N) ratio of the target. Standard array processing assumes that the signal arrives as a plane wave. Conventional beamforming uses the concept of delay and sum of received plane wave signals to estimate the target bearing. With the advent of matched field/mode processing it is possible to extend the detection range by exploiting the multipath arrivals of low frequency signals using, for example, a large aperture vertical or horizontal array.
Improved signal gain is obtained because matched field processing matches the data with signal propagation in the waveguide. Matched field processing may also be used for source localization. The parameter estimation aspect of the method has been extensively investigated. Assuming that the acoustic environment of the ocean is known and the signal can be modeled for all source ranges and depth of interest, the bearing, range, and depth of the target is estimated by the highest correlation point in the correlation ambiguity function. If the correlation is in terms of the mode amplitudes of the replica and data field, one has a matched-mode processing.
For a horizontal line array or spherical array, conventional beamforming has been widely used for detection and bearing estimation of a target. The highest beam yields the target bearing if the dominant arrivals of the signal are contained in one beam, as when the target look direction is near the broadside of the horizontal array. When the source is near an end fire direction, the multipath arrivals may spread the signal over several beams causing signal gain degradation and bias in direction estimation.
A horizontal line array is the preferred means for estimating the bearing of an underwater object emitting acoustic signals in an ocean waveguide. The array needs to have an aperture of typically greater than 5-10 wavelengths in order to provide a sufficient bearing resolution and processing gain. At ranges where the signal distortion due to random acoustic media has not completely destroyed the relative signal phase, the array aperture can be large and for this reason, a line array is most practical. While the bearing of the source (also referred to as target) can be estimated by conventional beamforming, it is also highly desirable to be able to estimate the range of the source.
To accomplish the latter objective, a modification of conventional beamforming has been proposed assuming a spherical curvature wave front for the emitted acoustic signal. This is called range-focused beamforming, as the curvature wave front depends on the source range. To estimate the target range, range-focused beamforming is applied to data assuming several hypothesized target ranges. The beam outputs with the highest intensities are used to estimate the target range. Range estimation is limited to a target at near broadside directions and at ranges less than the Fresnel zone. This approach breaks down at near end fire directions.
Another approach to sonar array processing is the so-called matched-field processing which was originally proposed for a vertical line array and has later been extended to a horizontal line array. Matched-field processing assumes a signal propagation model based on the physics of acoustic wave propagation in the ocean, which requires environmental acoustic inputs such as the sound speed profile in the water column, the bottom depth (or bathymetry for a range dependent model) and bottom sound speed, density and attenuation. For a hypothesized target range and depth, the modeled acoustic field (called the replica field) on a vertical array is correlated with the received data. The outputs are used to create a range-depth ambiguity surface where the peak is used to localize the target. Matched-field processing has been extended to a horizontal line array where for a given target bearing (as determined from conventional beamforming), one can search for target range and depth.
One critical issue for matched-field processing, which concerns both the vertical and horizontal array, is the environmental mismatch, which occurs when the environmental inputs used to create the replica field are uncertain and may incur errors. The results of environmental mismatch include range and depth errors which, depending on the nature of the mismatch, may be large.
Another way to perform the matched-field processing is matched-mode processing wherein the correlation of the replica field with the data field is carried out in the normal mode space. In acoustic propagation in an ocean waveguide, the field may be represented as a sum of many normal modes; the normal modes are eigen-functions of the waveguide, such as, for example, sinusoidal waves are eigen-modes of a guitar string. Using the mode expressions, the ability to estimate the range and depth for a vertical and horizontal line array can be quantitatively modeled.
In view of the fact that all the sonar array processing software has been developed based on conventional beamforming for a horizontal line array, a variation of matched field processing has been proposed using the preformed conventional beams as the input data. The beam data are correlated with the conventional beams formed from the replica field (the replica beams). This is called matched-beam processing. Matched-beam processing is equivalent to matched-field processing if all beams are used; this follows from the convolution theorem as conventional beamforming is a wave number transform of the sensor data. Based on the beam domain algorithm, a post sonar array processor has been proposed as an appendix to the conventional beamforming processor to estimate the target range and bearing.
Matched-beam processing can be interpreted as a beamforming technique as discussed below. Based on this formulism, it is unified with range-focused beamforming and shown to be a natural extension of the range-focused beamforming. As such, the processor performance analysis can be handled the same way as for range-focused beamforming.
While matched field processing has shown the capability to localize a source, in practice there often exist loud interference sources (e.g., surface ships) producing sidelobes in the ambiguity surface that may mask the target source or influence the source location. Adaptive signal processing has been developed (in the context of plane wave arrivals) to null the interference. But the performance is limited by signal mismatch between the data and model, and beam spreading due to multipath arrival. For example, the dominant mode rejection approach requires the interference signal (the dominant mode) to be orthogonal to the target signal, which is not always true in practice.
This invention employs two adaptive signal processing approaches to suppress the interference. One is to use the depth discrimination based on the fact that the submerged source couples to different normal modes than the surface interference. The other is to process the target source and interference using different beam sub-spaces assuming that they are detected on different beams by conventional means. The latter is the reduced beam space processor.
An apparatus for processing passive acoustic signals received on a horizontal line array that were either emitted from an underwater object or echo returned from an object, is proposed to display a radar-like range-bearing image of the object, thereby showing the location of the object relative to the receiver horizontal line array. The range-bearing images can be created at different water depth to search for an underwater object in a three-dimensional space. The method includes receiving an acoustic signal from the target, determining a beam covariance matrix, determining a replica field for a range-bearing of the target, determining a replica beam from the replica field; and processing the beam covariance matrix and the replica beam to determine range-bearing ambiguity surface and estimate depth ambiguity for selected peaks of the ambiguity surface.
The range-bearing image is focused to some particular depth. When focused to the depth of an underwater source, it has the capability of suppressing ship-radiated or surface-generated noise thereby increasing the detectability of the underwater source (henceforth called xe2x80x9ctargetxe2x80x9d) under a noisy shipping condition or foul weather condition. Signals can be passive signals radiated from the source or echo returns from an active source. The depth discrimination is achieved by exploiting the difference in the coupling of the acoustic source with the sound channel (acoustic normal modes) between sources of different depths; the difference is prominent in a downward refractive sound speed profile.
In one aspect, the present invention provides an apparatus for processing acoustic signals received on a horizontal line array for creating range-bearing images of a target, the apparatus includes a receiver array for receiving an acoustic signal from the target; a processor for determining a beam covariance matrix; the processor determining a replica field for a range-bearing of the target, the processor further used for determining a replica beam from the replica field. The processor further processes the beam covariance matrix and the replica beam to determine range-bearing ambiguity surface and estimate depth ambiguity for selected peaks of the ambiguity surface. The apparatus further comprises a display device for displaying a radar-like image of the target. The radar-like image is preferably created with instantaneous data, the data being continuously updated. The acoustic signals from the target are preferably passive signals radiated from the target or echo returns from an active source. In the above apparatus a sub-space of the beam covariance matrix is used in a target-search direction, thereby resulting in a reduced number of computations. The replica beam is determined from the replica field by transforming data from the target from a phone space to a beam space. The apparatus is preferably configured for passive searching of an underwater object and for active searching of an underwater object.
In another aspect, the present invention provides a method for processing acoustic signals received on a horizontal line array for creating range-bearing images of a target, the method comprising: receiving an acoustic signal from the target; determining a beam covariance matrix; determining a replica field for a range-bearing of the target; determining a replica beam from the replica field; processing the beam covariance matrix and the replica beam using a beam sub-space adaptive processor; producing a range-bearing ambiguity surface to determine the source range and bearing; and producing a depth ambiguity function to estimate the source depth for selected peaks of the ambiguity surface. A sub-space of the beam covariance matrix is preferably used in a target-look direction, thereby resulting in a reduced number of computations, a separation of the target sub-space from the interference sub-space, the interference contribution to the target sub-space being suppressed by peak-to-sidelobe-ratios provided by conventional beamforming. The replica beam is preferably determined from the replica field by transforming data from the target from a phone space to a beam space. Range errors due to environmental mismatch are preferably minimized by a proper choice of mode numbers, the choice being determined by the wave number spread of normal modes based on a database of sound speed profiles for a given area. The range estimation is preferably calibrated using, ocean-moving vehicles, such as for example, ships, of opportunities as a source. Range and bearing estimation of the source is preferably obtained initially using a hybrid processor based on a frequency independent set of weighting coefficients without a precise knowledge of depth of the source. The weighting coefficients represent mode amplitudes at a frequency lower than the frequency band of interest. Range estimation is further refined after depth is estimated using the initial range estimation.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.